Do both and you will have a bigger picture. 

If you are not able to do both, what you should choose, depends on your question.

try XRF test.

dissolution is better, but it is more difficult to carry out. you are going to use HF

Electroosmosis. Water in anode well and solution out in catode well.

NAA could be used on the dry and powdered samples.

Dear

You can follow either dissolved or total metal analysis. Total metal analysis is abroad analysis. 

I suggested the use of NAA for multi-elemental nature of the method. In this method because of the negative effects of the dissolved samples in a reactor environment only the dry samples are used.  

For trace element analysis, dissolution method commonly used and on comparison with various other methods and found better result. 

NAA has been used extensively for trace elements at ppm and ppb levels.It can be easily used for detection of about forty or more elements. It has very wide range of applications in pollution, environment, mining, metallurgy, archeology, agriculture etc.